Magnetic ball head speed governor

ABSTRACT

The magnetic biasing winding of a magnetic ball head speed governor is spaced radially of the flyweights for increased mechanical advantage which permits the speed of an associated prime mover to be adjusted over an appreciable range by adjustment of the magnetic bias level. The flyweights may comprise permanent magnets to provide a &#39;&#39;&#39;&#39;polarized&#39;&#39;&#39;&#39; governor or non-magnetized magnetic material to provide a &#39;&#39;&#39;&#39;non-polarized&#39;&#39;&#39;&#39; governor. The magnetic bias winding may be disposed radially inwardly or outwardly of the flyweights.

[451 Aug. 14, 11973 GOVERNOR Primary Examiner.lames J. Gill [75] Inventor: Donald L. Davis Rockford, L Attorney-Wolfe, Hubbard, Leydig, Volt & Osann Woodward Governor Company, Rockford, Ill. ABSTRACT June 14, 1972 The magnetic biasing winding of a magnetic ball head Appl' 262705 speed governor is spaced radially of the flyweights for increased mechanical advantage which permits the 73/518 speed of an associated prime mover to be adjusted over Gfllp 3/16, 605g 13/24 an appreciable range by adjustment of the magnetic [58] Field of Search.................... 73/518 bias level. The flyweights may comprise permanent magnets to provide a polarized governor or non- References Cited magnetized magnetic material to provide a non- U D STATES PATENTS polarized governor. The magnetic bias winding may 2,715,528 8/1955 73/518 beffisposed radially inwardly mwardly the 2,890,877 6/1959 Straznickas...........................73/518 Welghts- FOREIGN PATENTS OR APPLICATIONS 12 Claims, 5 Drawing Figures 53,077 4/1912 Austria....... 73/5l8 iJnited States Patent [191 Davis MAGNETIC BALL HEAD SPEED [73] Assignee:

[22] Filed:

[52] ILLS. Cl. [51] Int.

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BACKGROUND OF THE INVENTION This invention relates generally to governors and, more particularly, to magnetic ball head speed governors.

As is known, a ball head speed governor typically comprises flyweights fulerumed on a rotary ball head to swing about tangentially disposed pivots spaced radially outwardly from the axis of rotation of the ball head. A bias spring, which is selectively stressed manually, is normally provided to bias the flyweights towards the axis of rotation of the ball head so as to oppose the centrifugal force developed by the flyweights as a result of the ball head rotation. The spring bias is adjusted so that the centrifugal force, developed as the ball head is rotated by the prime mover, is precisely equal and opposite the spring force only if the prime mover is operating at its set point speed. Operation of the prime mover at any other speed causes an unbalanced condition to exist in the governor such that a control action is initiated to increase or decrease the speed of the prime mover back to the set point value.

It has previously been suggested that an additional magnetic biasing capability be included in ball head speed governors to permit more sensitive adjustment of the flyweight bias than can be achieved by manually adjusting the spring stress alone. The available magnetic ball head speed governors are not, however, fully satisfactory, especially in applications requiring appreciable variation of the bias on the flyweights by adjustment of the magnetic bias level as, for example, in the system disclosed and claimed in the commonly assigned, copending Leeson application Ser. No. 262,792 filed June 14, 1972 on Digital Synchronizing and Phasing System for Prime Movers. Indeed, the magnetic biasing capability of known magnetic ball head speed governors has been limited to use principally as a vernier adjustment. Also, the prior magnetic ball head speed governors have typically had a relatively low magnetic efficiency due to flux leakage.

SUMMARY OF THE INVENTION Accordingly, the primary aim of the present invention is to provide an improved magnetic ball head speed governor having a relatively high mechanical advantage for the magnetic bias applied to the flyweights. Another object is to provide a non-polarized, as well as a polarized governor of the foregoing type.

One of the more detailed objects in keeping with certain aspects of this invention is to provide polarized" and non-polarized" magnetic ball head speed governors which have relatively high magnetic efficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of this invention will become apparent as the following detailed description is read in conjunction with the attached drawings, in which:

FIG. I is a vertical section taken through the axis of rotation of a magnetic ball head speed governor embodying the present invention;

FIG-S. 2 and 3 are fragmentary cross sections respectively taken along the lines 2-2 and Et -3 in FIG. I;

FIG. 4 is a simplified, fragmentary vertical section of a governor embodying an alternative form of the present invention; and

FIG. 5 is an exploded view of a flyweight for a polarized governor.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS While the invention is hereinafter described in detail with reference to certain illustrated embodiments, it is to be understood that the intent is not to limit it to those embodiments. To the contrary, the intent is to cover all modifications, alternatives and equivalents falling within the spirit and scope of the invention as defined by the appended claims.

As shown in FIGS. 1-3, the invention is embodied in a governor generally similar to the one described and claimed in commonly assigned Drake US. Pat. No. 3,251,373. The governor may be employed, for example, to regulate the speed of a prime mover (not shown). More particularly, the governor comprises a hollow casing I0 defining a cavity 11 within which a ball head assembly 12 is housed. The ball head assembly is coupled by an internally splined clutch mechanism I3 to the upper end of a drive sleeve I4 which is splined as at IS. The lower end of the drive sleeve 14 is also splined as at 16 so that it may be readily coupled to the output shaft of the prime mover. Hence, in operation, the drive sleeve I4 and ballhead assembly I2 are rotated at a speed proportional to the speed of the prime mover.

The illustrated governor controls the speed of the prime mover by controlling the flow of pressure fluid, such as oil, to and from a speed regulating servo (not shown) which is coupled to the governor by means of a hydraulic coupling I7 and piping (also not shown). Thus, to provide the hydraulic control there is a tubular plunger III which is journaled in the drive sleeve I4) and which, together with the drive sleeve, defines a valve 19. The plunger 13 is slidable axially of the drive sleeve I4 under the control, of the governor as described more fully hereinbelow to selectively couple the hydraulic coupling I7 to a high pressure hydraulic source 21 and a drain cavity 22. The hydraulic source 2I suitably comprises a pump 23, which is driven by the drive sleeve I4, and a spring loaded regulating valve 24. As shown in FIG. I, the plunger I8 is in the central null position it assumes when the governor is balanced. If the governor is unbalanced by an underspeed condition of the prime mover, the plunger 18 is displaced downwardly to complete a path between the pressure source 2I and the speed regulating servo via the regulating valve 24, an arcuate slot 25 in the plunger which is then in registry with a port 26 in the drive sleeve, and an axially elongated slot 27 in the plunger which is in continuous communication with another drive sleeve port 28 leading to the hydraulic coupling I7. If, on the other hand, the governor is unbalanced by an overspeed condition of the prime mover, the plunger is displaced upwardly, thereby completing a path between the servo and the drain cavity 22 via the hydraulic coupling 17, the registered slots 27 and 28, and an arcuate slot 29 through the plunger which is then in registry with a drive sleeve port 30.

The ball head assembly I2 includes a channel 41 having a flat bottom engaged with the face of the clutch I3 and opposed parallel flanges 42 and 43. A pair of flyweights M and 45 are mounted on the ball head assembly at opposite sides of its axis of rotation by respective pivot pins 46 and 47 which project through holes in the flanges 42 and 43. The channel 41 and clutch mechanism are seated in a spring clip 51 which has opposed sidewalls 52 and 53 bearing respectively against the flanges 42 and 43 to prevent endwise displacement of the pivot pins. The spring clip is, in turn, seated in the base wall of the cavity 11.

The valve punger 18 extends upwardly into the cavity 11, and has its upper end rigid with an annular shoulder 61. The shoulder 61 rides on a bearing 62 which, in turn, rests on a washer 63. The flyweights 44 and 45 are generally L-shaped members comprising, as best seen in FIGS. 2 and 3, a pair of opposed arms 55 and 56 which extend radially inwardly from the lower end of a vertically extending flanged leg or web 54. The pivot pins 46 and 47 pass through the arms of the respective flyweights, and the vertically extending legs of the flyweights are slightly offset at their lower ends radially outwardly from the pivots. The longer arms 55 of the flyweights straddle the plunger 18 and underlie the washer 63.

The washer 63 is urged against the flyweight arms 55 by the bias of a speeder spring 64. As shown, the speeder spring has its lower end bearing against the annular shoulder 61 and its upper end secured to a rod 65 which is threaded through a casing cover 66 and connected at its upper end to a control lever 67. Hence, by rotating the lever, to adjust the rod upwardly or downwardly the spring compression may be varied to adjust the spring bias applied to the flyweights 44 and 45 via the shoulder 61, bearing 62 and washer 63. There is a clearance between the plunger 18 and the washer 63 to aid in preventing vibration from effecting the flyweights. Also, to define limits for the stressing of the speeder spring 64, a pin 68, which may be vertically adjustable relative to the rod 65 (by means not shown) but which in operation is fixed for movement with the rod 65 extends downwardly into a cup 69 formed at the extreme upper end of the plunger 18. The upper rim of the cup 69 has an inwardly extending flange 71 and the pin 68 has an annular shoulder 72 at its lower end. Thus, as the control lever 67 is rotated in one direction, a limit is reached when the rod 68 bottoms on the base of the cup 69. On the other hand, when the control lever is rotated in the opposite direction, a limit is reached when the shoulder 72 engages with the flange 71.

As thus far described, the governor is generally conventional. The outward centrifugal force developed in the flyweight legs applies a force, in magnitude a function of ball head speed, upwardly through the arms 55 to the washer 63 to oppose the downward bias of the speeder spring 64. Normally, when the centrifugal force is precisely equal and opposite the spring bias, the governor is balanced such that the flyweights 44 and 45 extend vertically substantially parallel to the axis of rotation of the ball head assembly 12 to thereby center the plunger 18 in its central null position. The greater the set point compression given to the spring 64 by adjusting the rod 65, the greater must be the speed of the drive sleeve 14 (and the controlled prime mover) to develop sufficient centrifugal force in the flyweights to restore the plunger 18 to its centered position.

In keeping with the present invention, the flyweights 44 and 45 are magnetically responsive to the magnetic bias created by energization of a coil 81 which is spaced radially of the flyweights. The flyweights may comprise permanent magnets, in which event the governor is polarized," or non-magnetized magnetic material such as soft iron, in which event the governor is non-polarized." In the former case, the magnetic bias acting on the flyweights has a sense dependent on the polarity of the magnetic field, whereas in the latter case, the magnetic bias on the flyweights has a sense independent of the field polarity. In other words, a polarized governor has a bi-directional magnetic response which may either add to or subtract from the force of the speeder spring; and a non-polarized governor has a unidirectional magnetic response which always acts in the same sense. The magnitude of the magnetic bias, on the other hand, depends in both types of governors on the magnetic field intensity or strength.

In carrying out this invention, the biasing coil 81 may be disposed either radially inwardly or radially outwardly of the flyweights. it is, however, important that the coil be in substantial radial alignment with the vertically extending legs of the flyweights to obtain the substantial mechanical advantage for the magnetic biasing which this invention contemplates. Because of the radial alignment between the biasing coil and the vertical legs of the flyweights there is a relatively long lever arm extending from the horizontal axis passing through the flyweight pivots to the upper ends of the flyweight legs to provide the desired mechanical advantage. This may be readily contrasted with prior art magnetic ball head speed governors such as shown in Straznickas US. Pat. No. 2,890,877 where the relatively short radial extensions of the flyweights limit the mechanical advantage to a relatively low level.

More particularly, the governor shown in FIGS. 1-3 is a non-polarized governor, and the magnetic biasing coil 81 is spaced radially inwardly of the flyweights 44 and 45. The coil 81 comprises numerous turns wound concentrically of the axis of rotation of the ball head assembly 12 on an annular core 82. The core 82, which may suitably be formed from iron or the like, is supported by a ring 83 and has upper and lower flanges 84 and 85 which extend outwardly over and under the windings to define magnetic pole pieces for the coil. The flyweights 44 and 45, on the other hand, may suitably be soft iron stampings. Hence, when the coil 81 is energized and magnetic flux established in the path shown by dashed lines in FIG. 1, the flyweights 44 and 45 are attracted toward the coil by a magnetic biasing force which, in this case, is cumulative with the spring biasing force. That is, the spring 64 pushes downwardly on the arms 55 and tends to rock the legs 54 inwardly about the pins 46 and 47, whereas the magnetic flux creates an attractive force on the legs 54 which similarly tends to rock them inwardly. The magnetic force is generally proportional to the excitation current supplied to the coil 81 and thus may be conveniently varied to change effective set point speed established jointly by the spring 64 and the magnetic effect of the coil 81. Thus, the governor is balanced only when the centrifugal force developed by the flyweights is equal to the sum of the spring and magnetic biasing forces or, in other words, the net bias on the flyweights.

There are stops (not shown) to limit the pivotal movement of the flyweights 44 and 45 so that they can not actually touch the bias coil 81 or the flanges 84, 85. Assuming the coil 81 is energized, it will be understood that there is then a magnetic field which attracts the flyweights toward the coil. The magnetic path for flux is shown by dashed lines as extending from the upper flange or pole piece 84 of the coil, through an air gap, down through the vertically extending web 54 of the flyweight, and back through another air gap to the pole piece 85 of the coil. The magnetic biasing force depends on the amount of flux linking the flyweightsto the coil which, in turn, depends on the reluctance of the magnetic paths through which the fiux flows and the intensity of the magnetic field. The field intensity, of course, depends on the number of windings comprised by the coil, together with the amount of current passing through the coil. By varying the current, the effective set point may be changed over a wide range of speed values.

Preferably, in keeping with one aspect of this invention, provision is made to maintain the reluctance of the magnetic paths between the coil 81 and the flyweights 44 and 45 substantially constant so that the magnetic force applied to' the flyweights is substantially constant regardless of changes in the position of the flyweights. The lengths of the air gaps between the upper pole piece or flange 84 and the flyweights 44 and 45 tend to vary as the flyweights pivot. As will be appreciated, changes in those air gaps are undesirable because such changes cause the reluctance of the magnetic paths for the flux linking the flyweights to the coil to vary and, therefore, cause the magnetic forces on the flyweights to change as a function of the speed of rotation of the ball head assembly l2. To reduce the variations of reluctance, the coil 18 is made frusto-conical in shape, tapering from a relatively large lower diameter to a smaller upper diameter. Moreover, to accommodate the frusto-conical shape of the coil without requiring undesirably large air gaps between the upper end of the coil and the flyweights, the flyweights are relieved as at 86 and 87 opposite the lower end of the coil.

Further, in keeping with another aspect of the present invention, a relatively high magnetic efficiency is achieved by spacing the magnetic biasing coil 81 radially inwardly of the flyweights 44 and 45. Specifically, because the casing is hollow to provide the cavity 1 1 required for rotation of the ball head assembly 12 and pivotal movement of the flyweights 44 and 45, it is feasible to mount the coil 81 so that it is magnetically isolated from substantially all parts of the governor (except for the flyweights 44 and 45) by relatively large air gaps. The large air gaps insure that the alternative paths for flux have relatively high magnetic reluctances and are, therefore, effective to minimize undesirable flux leakages. it will, of course, be understood that such isolation is not complete because of the necessity for supporting the core 82 as by the ring 83. But, there is sufficient isolation to insure that the governor has a relatively high magnetic efficiency, and thus may be enhanced by making the ring 83 of a paramagnetic material such as brass.

Referring to the alternative embodiment of the invention shown in FIGS. 4 and 5, it will be seen that the flyweights 44a and 45a comprise permanent magnets and that the coil 81a is spaced radially outwardly of the flyweights. As will be appreciated the alternative embodiment shown is an example of a polarized governor.

The coil 810 comprises numerous turns wound concentrically of the axis of rotation of the ball head assembly on a non-magnetic ring-like core 9], which may suitably be formed from aluminum. Encasing the upper and lower sides of the coil 81a there are respective annular pole pieces 92 and 93 which, in turn, are fixed to the upper and lower ends of a ring-like pole piece 94 surrounding the outer side of the coil. The pole pieces 92-94 are made of a magnetic material, such as iron. lf current is passed through the coil in one direction, the upper pole piece 92 becomes a north pole and the lower pole piece 93 is a south pole, but if the current flows through the coil in the opposite direction, the magnetic polarization is reversed such that the upper pole piece 92 becomes a south pole and the lower pole piece 93 is a north pole.

The flyweights 44a and 45a constructed with permanent magnet elements have fixed magnetic polarizations. In keeping with a specific feature of this invention, the fabrication of the flyweights may be simplified by using an interlocking construction. More particularly, as shown in FIG. 5, each of the flyweights may advantageously comprise a generally L-shaped, nonmagnetic cradle 95 having a vertically extending leg 96 which is slotted as at 97. The slotted leg 96 permits a permanent magnet 98 to be seated between upper and lower pole pieces 99 and 100 which have their opposite sides slotted to fit over and interlock with the posts 101 and 102 provided by the leg 96. Further, the permanent magnet 98 may have a T-shaped cross-section such that it may be pressed into the slot 97 to overlie the inner face of the leg 96. Suitably, the cradle 95 may be a nonmagnetic stainless steel stamping, the permanent magnet 98 may be a casting, the pole pieces 99 and 100 may be magnetic ingot iron stampings, and a bond between the cradle 95 and permanent magnet 98 may be provided by a high tensile strength adhesive or brazing.

As a result of the fixed magnetic polarization of the flyweights 44a and 45a, the magnetic bias applied to them has a sense dependent on the magnetic field polarity and a magnitude dependent on the magnetic field intensity. For example, assume that the flyweights are polarized so that they have north poles at their upper ends and south poles at their lower ends. In that event, if the direction of current flow through the coil 81a is such that the resultant magnetic field causes the pole piece 92 to be a north pole and the pole piece 93 to be a south pole, the flyweights 44a and 45a will be repelled (urged inwardly about the pivot pins, as viewed in FIG. 4) by a force which is additive with the bias force from the spring 64. On the other hand, if the direction of current flow through the coil 81a is reversed so that the pole piece 92 is a south pole and the pole piece 93 is a north pole, the flyweights 44a and 45a will be attracted or biased toward the coil by a force which is subtractive from the spring bias. Hence, it will be understood that the net flyweight bias, i.e., the algebraic summation of the magnetic bias and spring bias on the flyweights 44a and 45a, may be increased and decreased by varying the amplitude and polarity of the current flow through the coil 81a.

There are stops (not shown) to prevent the flyweights 44a and 45a from engaging the pole pieces 92 and 93. Also, the face of the pole piece 92 may be beveled as at 103 to maintain the reluctance of the magnetic paths or more nearly constant as the flyweights move and the lengths of the air gaps change slightly. As will be seen by dashed lines in FIG. 4, the magnetic paths may be traced from the upper pole pieces 99 of the flyweights, through air gaps to the upper pole piece 92 of the coil, then through the outer and lower pole pieces 94 and 93 of the coil, and back through air gaps to the lower pole pieces 100 of the flyweights. If the flux contributed by the energization of the coil 81a passes in the same direction as the flux contributed by the permanent magnets 98 of the flyweights, the flyweights are biased toward the coil. If, however, the flux produced by energization of the-coil 810 bucks or opposes the flux provided by the permanent magnets 98, the flyweights are biased away from the coil.

CONCLUSION It will now be understood that a magnetic ball head speed governor with a relatively large mechanical advantage for the magnetic bias applied thereto has been provided. In particular, it will be appreciated that the flyweights provide lever anns of appreciable length which permit the net bias on the flyweights to be varied over a substantial range by changing the magnitude of exciting current supplied to the coil. Further, it will be appreciated that the magnetic ball head speed governor provided by this invention, especially the governor having the biasing coil spaced radially inwardly of the flyweights, has a relatively high magnetic efficiency.

I claim as my invention:

1. A magnetic ball head speed governor comprising the combination of a hollow casing defining a cavity;

a ball head assembly mounted for rotation in said cavity about a predetermined axis of rotation;

a pair of magnetically responsive flyweights mounted on said ball head assembly at opposite sides of said axis of rotation for pivotal movement toward and away from said axis of rotation about respective pivot axes, each of said flyweights having an arm extending inwardly toward said axis of rotation and a leg extending generally parallel to said axis of rotation along a line offset outwardly of said pivot axis;

a control means engaged with the arms of said flyweights; and

a magnetic biasing coil supported by said housing concentrically with said axis of rotation, said coil being spaced from but in radial alignment with said flyweights, whereby said flyweights are magnetically biased in response to energization of said coil by current flow.

2. The governor of claim 1 further including a spring bearing against said control member to maintain it in engagement with said flyweights and to apply a further bias to said flyweights.

3. The governor of claim 2 wherein the flyweights comprise non-magnetized magnetic material so that the magnetic bias applied to said flyweights has a magnitude which varies as a function of variations in the amplitude of said current flow but a sense which is indeas a function of variation in the amplitude of said current flow and a sense dependent on the polarity of the current flow.

5. The governor of claim 4 wherein each of said flyweights comprise a non-magnetic cradle having a slotted leg, upper and lower pole pieces interlocked with said leg, and a permanent magnet seated between said pole pieces and secured to said leg.

6. The governor of claim 2 wherein said coil is spaced radially outwardly from said flyweights.

7. The governor of claim 2 wherein said coil is spaced radially inwardly of said flyweights and is magnetically isolated from substantially all other parts of said governor by relatively large air gaps.

8. In a hydraulic speed governor having a ball head assembly, a valve including a sleeve coupled to rotate said ball head assembly about a predetermined axis of rotation and a plunger slidable in said sleeve and extending through said ball head assembly, an annular shoulder on said plunger, a pair of flyweights pivotally mounted on said ball head assembly on opposite sides of said axis for rotation toward and away from said axis with each of said flyweights including a leg extending generally parallel to but spaced from said axis of rotation and an arm extending inwardly from one end of said leg into engagement with said shoulder, and a spring bearing on the opposite side of said shoulder to hold it in engagement with the arms of said flyweights; the improvement comprising a coil for magnetically biasing said flyweights, said coil being concentric with said axis of rotation and being mounted in radial alignment with the legs of said flyweights.

9. The improvement of claim 8 wherein the coil is spaced from said flyweights by air gaps and is shaped so that the reluctance of the air gaps remains substantially constant as the flyweights rotate toward and away from said axis.

10. The improvement of claim 9 wherein the coil is spaced radially inwardly of the flyweights and has a frusto-conical shape.

11. The improvement of claim 9 wherein the coil is spaced radially outwardly of the flyweights and has a beveled pole piece adjacent the outer ends of said flyweights.

12. The improvement of claim 10 wherein each of said flyweights comprise a non-magnetic cradle having a slotted leg, upper and lower pole pieces interlocked with said leg, and a permanent magnet seated between said pole pieces and secured to said leg.

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1. A magnetic ball head speed governor comprising the combination of a hollow casing defining a cavity; a ball head assembly mounted for rotation in said cavity about a predetermined axis of rotation; a pair of magnetically responsive flyweights mounted on said ball head assembly at opposite sides of said axis of rotation for pivotal movement toward and away from said axis of rotation about respective pivot axes, each of said flyweights having an arm extending inwardly toward said axis of rotation and a leg extending generally parallel to said axis of rotation along a line offset outwardly of said pivot axis; a control means engaged with the arms of said flyweights; and a magnetic biasing coil supported by said housing concentrically with said axis of rotation, said coil being spaced from but in radial alignmeNt with said flyweights, whereby said flyweights are magnetically biased in response to energization of said coil by current flow.
 2. The governor of claim 1 further including a spring bearing against said control member to maintain it in engagement with said flyweights and to apply a further bias to said flyweights.
 3. The governor of claim 2 wherein the flyweights comprise non-magnetized magnetic material so that the magnetic bias applied to said flyweights has a magnitude which varies as a function of variations in the amplitude of said current flow but a sense which is independent of the polarity of the current flow.
 4. The governor of claim 2 wherein the flyweights comprise permanent magnets so that the magnetic bias applied to said flyweights has a magnitude which varies as a function of variation in the amplitude of said current flow and a sense dependent on the polarity of the current flow.
 5. The governor of claim 4 wherein each of said flyweights comprise a non-magnetic cradle having a slotted leg, upper and lower pole pieces interlocked with said leg, and a permanent magnet seated between said pole pieces and secured to said leg.
 6. The governor of claim 2 wherein said coil is spaced radially outwardly from said flyweights.
 7. The governor of claim 2 wherein said coil is spaced radially inwardly of said flyweights and is magnetically isolated from substantially all other parts of said governor by relatively large air gaps.
 8. In a hydraulic speed governor having a ball head assembly, a valve including a sleeve coupled to rotate said ball head assembly about a predetermined axis of rotation and a plunger slidable in said sleeve and extending through said ball head assembly, an annular shoulder on said plunger, a pair of flyweights pivotally mounted on said ball head assembly on opposite sides of said axis for rotation toward and away from said axis with each of said flyweights including a leg extending generally parallel to but spaced from said axis of rotation and an arm extending inwardly from one end of said leg into engagement with said shoulder, and a spring bearing on the opposite side of said shoulder to hold it in engagement with the arms of said flyweights; the improvement comprising a coil for magnetically biasing said flyweights, said coil being concentric with said axis of rotation and being mounted in radial alignment with the legs of said flyweights.
 9. The improvement of claim 8 wherein the coil is spaced from said flyweights by air gaps and is shaped so that the reluctance of the air gaps remains substantially constant as the flyweights rotate toward and away from said axis.
 10. The improvement of claim 9 wherein the coil is spaced radially inwardly of the flyweights and has a frusto-conical shape.
 11. The improvement of claim 9 wherein the coil is spaced radially outwardly of the flyweights and has a beveled pole piece adjacent the outer ends of said flyweights.
 12. The improvement of claim 10 wherein each of said flyweights comprise a non-magnetic cradle having a slotted leg, upper and lower pole pieces interlocked with said leg, and a permanent magnet seated between said pole pieces and secured to said leg. 